The organic resin-coated metal plate obtained by coating a metal blank such as of aluminum with an organic resin has long been known as a material for producing cans. It has also been well known to put a laminate thereof to the draw working or the draw ironing working to produce seamless cans for containing beverage, or to put the laminate thereof to the press forming to produce can lids such as easy-open ends.
In the organic resin-coated metal plate, however, adhesion property is not enough between the aluminum plate and the organic resin coating, or the corrosion resistance of the aluminum plate is not enough. Therefore, it is a practice to treat the surface of the aluminum plate with an inorganic or organic surface-treating agent. For instance, there has been known a chromic phosphate type surface-treating material which in the form of a single layer exhibits excellent corrosion resistance as well as excellently adhesion property when various organic resins are applied thereon. Many of the chromate treatments now utilized are those of the type that do not leave hexavalent chromium in the final products. However, the treating solution contains hexavalent chromium which is a toxic substance. Besides, after the disposal, it is worried that chromium might elute out into the soil. Therefore, it has been urged to develop a chromium-free surface treatment.
Various kinds of chromium-free surface treatments have been proposed for the materials for producing cans. As the chromium-free surface treatments for the aluminum alloy type metal plates, there have been proposed a method of forming a conversion-coated layer comprising an zirconium oxide and/or titanium as a main component on the surface of the aluminum-containing metal material by using an acidic treating solution that contains zirconium, titanium or a compound thereof, phosphate and fluoride and that has a pH of about 1.0 to 4.0 (patent document 1), and a method of forming an organic-inorganic composite layer containing an organic compound of carbon as a main component, a phosphorus compound and a zirconium compound or a titanium compound (patent document 2). Further, the present applicant has also proposed a surface-treated metal material based on a chromium-free treatment using Zr, O and F as main components but without using phosphoric acid ions, the chromium-free treatment being applicable to aluminum plates, steel plates as well as to containers (patent document 3).